1. Field of the Invention
This invention relates to antennas for radar and communication systems, and more particularly, to phased array antennas and their controllers used in radar and communication systems. This invention also relates to fiber optic corporate feed networks for use in phased array antennas.
2. Discussion of the Prior Art
The advent of wireless forms of communication necessitated the need for antennas. Antennas are required by every communication and radar system, and depending upon the specific application, antennas can be required for both receiving and transmitting signals. Antennas performing both receiving and transmitting functions are particularly important in radar and point-to-point communications. Early stages of wireless communication consisted of transmitting and receiving signals at lower frequencies, typically below 1 MHz, which resulted in signal wavelengths greater than 0.3 km. If the physical size of the antenna was not at least equal to the signal wavelength, then the antenna was not capable of directional transmission or reception. In more modern forms of wireless communication, such as with communication satellites, the frequency range of transmitted signals have shifted to the microwave spectrum where signal wavelengths are in the 1.0 cm to 30.0 cm range. Therefore, antennas can be practically realized with sizes much greater then the signal wavelength resulting in highly directional and narrow beams of radiation.
In radar systems the antenna must be capable of determining a targets direction, and to accomplish this requires a well defined narrow beam of radiation that searches for the target. Narrow radiation beams are created by antennas that are large compared to the signal wavelength. In addition, in order to be able to track or search for a target, the beam must be able to be moved in at least one direction, but preferably in two directions. Movement of the beam was typically accomplished by mechanically rotating a reflector; however, fixed array and phased array antennas provide a more feasible electronic means for scanning purposes.
Today virtually every radar and many communications system antennas have requirements for high directivity, high angular resolution and the ability to electronically scan or be reconfigured. These functions are typically accomplished using a phased array antenna with a large number of elements. An antenna array is a collection of two or more identical antennas that are closely arranged in a predetermined pattern and energized to produce high gain beams in specific directions. When antennas are combined in an array, constructive radiation interference results in a main beam of highly concentrated radiation, while destructive radiation interference outside the main beam reduces stray radiation. In order to produce the desired radiation patterns the individual radiating elements or antennas are characterized and each element is energized with the proper phase and amplitude relative to the other elements in the array. Basically, the antenna array exploits the interference that results from an array of many individual radiators which results in high gain while minimizing stray radiation.
With the elements of an array antenna closely spaced, it is preferable, if possible to feed all elements of the array with a single transmission line. One particular method of feeding all elements from a single transmission line involves using the corporate feed technique. In the corporate feed technique, all elements are fed in parallel using power splitters and equal delay transmission lines. The phase shift between elements can be controlled by phase shifters, and if the phase shifts can be varied, the array antenna becomes a phased array antenna. In a phased array antenna where each element has phase adjustability, the beam may be scanned by application of the appropriate phase gradient.
The following references disclose techniques for controlling and operating phased array antennas. The article "Optical Beam Control of mm-Wave Phased Array Antennas For Communications" by A. Dargoush et al., Microwave Journal, March 1987 discloses fiber optic links for the distribution of control and communications signals in a phased array system. The means and process disclosed in this article are for a fiber optic link which utilizes electronic multiplication techniques as opposed to the heterodyning and modulation techniques for up and down conversion as employed by the present invention.
The article "Distributed Beamsteering Control of Phased Array Radars" by T. P. Waldron et al., Microwave Journal, September 1986 discloses a transmitter/receiver module with a distributed control concept for the control of phased array antennas; however, the article does not disclose a transmitter/receiver module employing integrated fiber optic techniques as well as integrated electronic techniques in a single compact package.